1. Field of the Invention
This invention relates in general to the field of antennas. More particularly, the invention relates to dual-mode, common-aperture antenna systems that transmit and/or receive electromagnetic radiation in at least two frequency bands.
2. Description of the Prior Art
Radar is an active system which has been used extensively for detecting and determining the range and direction of distant objects such as ships and aircraft. Radar does this by illuminating the object with radiation and then receiving, analyzing and displaying the reflections. Many modern radar systems have sufficient resolving power to permit the identification of an object by analyzing its characteristic reflected pattern, or signature, as displayed by detection and classification equipment. In general, the resolution of a radar system and, therefore, its ability to identify objects from its signature increases as the operating frequency increases.
The ability of radar systems to effectively illuminate an object and receive a useful reflection can also vary with its operating frequency. For example, radar operation is often impacted by adverse weather conditions that can significantly alter the electromagnetic transmittance of the atmosphere. Specifically, while dense fog can have little effect on a microwave radar beam, it can quickly attenuate the short-wavelength beam of, for example, a laser radar. Those skilled in the art have therefore recognized that while most high-resolution radar systems produce good object-identification signatures, they can have limited ability at finding and/or illuminating objects under, for example, adverse weather conditions. Conversely, while many low-resolution radar systems may produce poor object-identification signatures, they have superior capacity at quickly finding objects under most operating conditions.
Consequently, in the field of object detection and identification, it has been found desirable to employ multi-mode radar systems that transmit and/or receive radiation in a number of frequency bands. These multi-mode radar systems have important applications in various apparatus such as aircraft landing systems, target acquisition and guidance equipment in smart bombs, obstacle detection radar for high-speed trains, marine navigation equipment, vehicle collision avoidance systems, and the like. In many of these applications, the radar system must be mounted in an apparatus having limited room and/or be capable of tolerating high acceleration forces. As such, one of the most critical problems confronting designers of multi-mode radar systems has been the low-cost fabrication of efficient, dual-mode antennas that are simple, compact and sturdy. The present invention fulfills this need.